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A little shame, and a little good luck -- BMS repair

MostlyHarmless

New Member
Joined
Jun 19, 2020
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6
When building my 7s Li-Ion battery, I numbered the cells 1-7 so I could keep track of them during assembly. When I received my BMS, I dutifully wired the individual cell tap wires to cells 1-7 according to the directions -- and double/triple checked to make sure they were all correct before plugging the harness into the BMS. As soon as I made the connection, I saw a puff of magic smoke and knew I messed up somewhere.

Here's the shame part: As this was all new to me, I didn't consider that my cells might be numbered opposite of what the BMS instructions had. This was the case -- which resulted in the full 24+ volts going through the first cell's input (for a 3.7v cell). Correcting the wiring to the proper cell order, I found that the BMS was not reporting the correct voltage for cell 1... understandable, since I had put a little more voltage into that line than it was designed for. But the other cells all reported proper voltages, so there might be some hope.

Putting the board under a microscope, I could see that one of the surface mount transistors had indeed been toasted, but all other components looked OK. Again with the microscope (my eyes can't see that close, anymore), I was able to read a number on one of the remaining good transistors, and ordered a few from Mouser Electronics. At only 14 cents apiece, I ordered 20 of them in case I couldn't get the part soldered in correctly the first few tries (surface mount components are a little tough to manage with conventional soldering equipment).

Now for the good luck: The new parts arrived a few days later -- the transistor in question is shown in the first pic. Pretty tiny, ain't it? Taking great care to remove the bad part, then clean and prep the board, I soldered in the new transistor; apparently correctly on the first try! The new transistor (circled on the second pic) seemed to be in place.

Hooking it all up (and making sure that the voltage taps were wired correctly, this time), I activated the BMS, connected to my phone, and was amazed to see that all seemed to be working properly!

I'm still testing, but think I dodged a bullet. Not only did I save myself a good chunk of money, but (more importantly), I didn't have to wait 2 months to get a new BMS.*

* As a side note, why is it so hard to find these sorts of components in the US?
 

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When building my 7s Li-Ion battery, I numbered the cells 1-7 so I could keep track of them during assembly. When I received my BMS, I dutifully wired the individual cell tap wires to cells 1-7 according to the directions -- and double/triple checked to make sure they were all correct before plugging the harness into the BMS. As soon as I made the connection, I saw a puff of magic smoke and knew I messed up somewhere.

Here's the shame part: As this was all new to me, I didn't consider that my cells might be numbered opposite of what the BMS instructions had. This was the case -- which resulted in the full 24+ volts going through the first cell's input (for a 3.7v cell). Correcting the wiring to the proper cell order, I found that the BMS was not reporting the correct voltage for cell 1... understandable, since I had put a little more voltage into that line than it was designed for. But the other cells all reported proper voltages, so there might be some hope.

Putting the board under a microscope, I could see that one of the surface mount transistors had indeed been toasted, but all other components looked OK. Again with the microscope (my eyes can't see that close, anymore), I was able to read a number on one of the remaining good transistors, and ordered a few from Mouser Electronics. At only 14 cents apiece, I ordered 20 of them in case I couldn't get the part soldered in correctly the first few tries (surface mount components are a little tough to manage with conventional soldering equipment).

Now for the good luck: The new parts arrived a few days later -- the transistor in question is shown in the first pic. Pretty tiny, ain't it? Taking great care to remove the bad part, then clean and prep the board, I soldered in the new transistor; apparently correctly on the first try! The new transistor (circled on the second pic) seemed to be in place.

Hooking it all up (and making sure that the voltage taps were wired correctly, this time), I activated the BMS, connected to my phone, and was amazed to see that all seemed to be working properly!

I'm still testing, but think I dodged a bullet. Not only did I save myself a good chunk of money, but (more importantly), I didn't have to wait 2 months to get a new BMS.*

* As a side note, why is it so hard to find these sorts of components in the US?
Good for you ! Most would have tossed it .
 
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